62
62

Sep 21, 2013
09/13

by
Maryam Arabsalmani; Varun Sahni; Tarun Deep Saini

texts

#
eye 62

#
favorite 0

#
comment 0

Future space-based gravity wave experiments such as the Big Bang Observatory (BBO), with their excellent projected, one sigma angular resolution, will measure the luminosity distance to a large number of gravity wave (GW) sources to high precision, and the redshift of the single galaxies in the narrow solid angles towards the sources will provide the redshifts of the gravity wave sources. One sigma BBO beams contain the actual source only in 68 per cent cases; the beams that do not contain the...

Source: http://arxiv.org/abs/1301.5779v2

44
44

Sep 18, 2013
09/13

by
Somnath Bharadwaj; Shiv K. Sethi; Tarun Deep Saini

texts

#
eye 44

#
favorite 0

#
comment 0

The emission from neutral hydrogen (HI) clouds in the post-reionization era (z < 6), too faint to be individually detected, is present as a diffuse background in all low frequency radio observations below 1420 MHz. The angular and frequency fluctuations of this radiation (~ 1 mK) is an important future probe of the large scale structures in the Universe. We show that such observations are a very effective probe of the background cosmological model and the perturbed Universe. In our study we...

Source: http://arxiv.org/abs/0809.0363v1

36
36

Sep 18, 2013
09/13

by
Tarun Deep Saini; Somak Raychaudhury

texts

#
eye 36

#
favorite 0

#
comment 0

We develop an algorithm for the reconstruction of the two-dimensional mass distribution of a gravitational lens from the observable distortion of background galaxies. From the measured reduced shear, the lens mapping is obtained, from which a mass distribution is derived. This is unlike other methods where the convergence ("kappa") is directly obtained. We show that this method works best for sub-critical lenses, but can be applied to a critical lens away from the critical lines. For...

Source: http://arxiv.org/abs/astro-ph/0002416v2

4
4.0

Jun 29, 2018
06/18

by
Mamta Gulati; Tarun Deep Saini

texts

#
eye 4

#
favorite 0

#
comment 0

We present a modal analysis of instabilities of counter-rotating, self-gravitating collisionless stellar discs, using the recently introduced modified WKB formulation of spiral density waves for collisionless systems (Gulati \& Saini). The discs are assumed to be axisymmetric and in coplanar orbits around a massive object at the common center of the discs. The mass in both discs is assumed to be much smaller than the mass of the central object. For each disc, the disc particles are assumed...

Topics: Astrophysics, Astrophysics of Galaxies

Source: http://arxiv.org/abs/1602.07169

7
7.0

Jun 29, 2018
06/18

by
Mamta Gulati; Tarun Deep Saini

texts

#
eye 7

#
favorite 0

#
comment 0

The short--wave asymptotics (WKB) of spiral density waves in self-gravitating stellar discs is well suited for the study of the dynamics of tightly--wound wavepackets. But the textbook WKB theory is not well adapted to the study of the linear eigenmodes in a collisionless self-gravitating disc because of the transcendental nature of the dispersion relation. We present a modified WKB of spiral density waves, for collisionless discs in the epicyclic limit, in which the perturbed gravitational...

Topics: Astrophysics, Astrophysics of Galaxies

Source: http://arxiv.org/abs/1601.04148

3
3.0

Jun 29, 2018
06/18

by
Soumavo Ghosh; Tarun Deep Saini; Chanda J. Jog

texts

#
eye 3

#
favorite 0

#
comment 0

Low surface brightness (LSB) galaxies are dominated by dark matter halo from the innermost radii; hence they are ideal candidates to investigate the influence of dark matter on different dynamical aspects of spiral galaxies. Here, we study the effect of dark matter halo on grand-design, m = 2, spiral modes in a galactic disk, treated as a collisionless system, by carrying out a global modal analysis within the WKB approximation. First, we study a superthin, LSB galaxy UGC 7321 and show that it...

Topics: Astrophysics, Astrophysics of Galaxies

Source: http://arxiv.org/abs/1606.08543

2
2.0

Jun 28, 2018
06/18

by
Soumavo Ghosh; Tarun Deep Saini; Chanda J. Jog

texts

#
eye 2

#
favorite 0

#
comment 0

Low surface brightness (LSB) galaxies form a major class of galaxies, and are characterized by low disc surface density and low star formation rate. These are known to be dominated by dark matter halo from the innermost regions. Here we study the role of dark matter halo on the grand-design, $m=2$, spiral modes in a galactic disc by carrying out a global mode analysis in the WKB approximation. The Bohr-Sommerfeld quantization rule is used to determine how many discrete global spiral modes are...

Topics: Astrophysics, Astrophysics of Galaxies

Source: http://arxiv.org/abs/1511.05536

34
34

Sep 18, 2013
09/13

by
A. Das; Shashikant Gupta; Tarun Deep Saini; Sayan Kar

texts

#
eye 34

#
favorite 0

#
comment 0

We investigate the case of a homogeneous tachyon field coupled to gravity in a spatially flat Friedman-Robertson-Walker spacetime. Assuming the field evolution to be exponentially decaying with time we solve the field equations and show that, under certain conditions, the scale factor represents an accelerating universe, following a phase of decelerated expansion. We make use of a model of dark energy (with p=-\rho) and dark matter (p=0) where a single scalar field (tachyon) governs the...

Source: http://arxiv.org/abs/astro-ph/0505509v2

118
118

Jul 20, 2013
07/13

by
Mamta Gulati; Tarun Deep Saini; S. Sridhar

texts

#
eye 118

#
favorite 0

#
comment 0

We study the linear $m=1$ counter-rotating instability in a two-component, nearly Keplerian disc. Our goal is to understand these \emph{slow} modes in discs orbiting massive black holes in galactic nuclei. They are of interest not only because they are of large spatial scale--and can hence dominate observations--but also because they can be growing modes that are readily excited by accretion events. Self-gravity being nonlocal, the eigenvalue problem results in a pair of coupled integral...

Source: http://arxiv.org/abs/1203.2239v1

47
47

Sep 18, 2013
09/13

by
Tarun Deep Saini; Somak Raychaudhury; Yuri A. Shchekinov

texts

#
eye 47

#
favorite 0

#
comment 0

Supernovae in distant galaxies that are gravitationally lensed by foreground galaxy clusters make excellent cosmological candles for measuring quantities like the density of the Universe in its various components and the Hubble constant. Distant supernovae will be more easily detectable since foreground cluster lenses would magnify such supernovae by up to 3-4 magnitudes. We show that in the case of the lens cluster Abell 2218, the detectability of high-redshift supernovae is significantly...

Source: http://arxiv.org/abs/astro-ph/0002448v1

44
44

Sep 19, 2013
09/13

by
Shashikant Gupta; Tarun Deep Saini; Tanmoy Laskar

texts

#
eye 44

#
favorite 0

#
comment 0

The most detailed constraints on the accelerating expansion of the universe and details of nature of dark energy are derived from the high redshift supernova data, assuming that the errors in the measurements are Gaussian in nature. There is a possibility that there are direction dependent systematics in the data, either due to uncorrected, known physical processes or because there are tiny departures from the cosmological principle, making the universe slightly anisotropic. To investigate this...

Source: http://arxiv.org/abs/astro-ph/0701683v2

40
40

Sep 18, 2013
09/13

by
Tarun Deep Saini; Somnath Bharadwaj; Shiv Kumar Sethi

texts

#
eye 40

#
favorite 0

#
comment 0

We investigate the possibility of detecting HI emission from gravitationally lensed HI clouds (akin to damped Lyman-$\alpha$ clouds) at high redshift by carrying out deep radio observations in the fields of known cluster lenses. Such observations will be possible with present radio telescopes only if the lens substantially magnifies the flux of the HI emission. While at present this holds the only possibility of detecting the HI emission from such clouds, it has the disadvantage of being...

Source: http://arxiv.org/abs/astro-ph/0101228v2

68
68

Sep 22, 2013
09/13

by
Roy Maartens; Varun Sahni; Tarun Deep Saini

texts

#
eye 68

#
favorite 0

#
comment 0

We examine the behavior of an anisotropic brane-world in the presence of inflationary scalar fields. We show that, contrary to naive expectations, a large anisotropy does not adversely affect inflation. On the contrary, a large initial anisotropy introduces more damping into the scalar field equation of motion, resulting in greater inflation. The rapid decay of anisotropy in the brane-world significantly increases the class of initial conditions from which the observed universe could have...

Source: http://arxiv.org/abs/gr-qc/0011105v3

97
97

Jul 19, 2013
07/13

by
Tarun Deep Saini; Shiv K. Sethi; Varun Sahni

texts

#
eye 97

#
favorite 0

#
comment 0

By observing mergers of compact objects, future gravity wave experiments would measure the luminosity distance to a large number of sources to a high precision but not their redshifts. Given the directional sensitivity of an experiment, a fraction of such sources (gold plated -- GP) can be identified optically as single objects in the direction of the source. We show that if an approximate distance-redshift relation is known then it is possible to statistically resolve those sources that have...

Source: http://arxiv.org/abs/1005.4489v1

45
45

Jul 20, 2013
07/13

by
Tarun Deep Saini; Somak Raychaudhury; Varun Sahni; A. A. Starobinsky

texts

#
eye 45

#
favorite 0

#
comment 0

Observations of high-redshift supernovae indicate that the universe is accelerating. Here we present a {\em model-independent} method for estimating the form of the potential $V(\phi)$ of the scalar field driving this acceleration, and the associated equation of state $w_\phi$. Our method is based on a versatile analytical form for the luminosity distance $D_L$, optimized to fit observed distances to distant supernovae and differentiated to yield $V(\phi)$ and $w_\phi$. Our results favor...

Source: http://arxiv.org/abs/astro-ph/9910231v4

10
10.0

Jun 28, 2018
06/18

by
Naveen Jingade; Tarun Deep Saini; Scott Tremaine

texts

#
eye 10

#
favorite 0

#
comment 0

We have examined the effect of slow growth of a central black hole on spherical galaxies that obey S\'ersic or $R^{1/m}$ surface-brightness profiles. During such growth the actions of each stellar orbit are conserved, which allows us to compute the final distribution function if we assume that the initial distribution function is isotropic. We find that black-hole growth leads to a central cusp or ``excess light', in which the surface brightness varies with radius as $R^{-1.3}$ (with a weak...

Topics: Astrophysics, Astrophysics of Galaxies

Source: http://arxiv.org/abs/1508.06409

42
42

Jul 19, 2013
07/13

by
Shashikant Gupta; Tarun Deep Saini

texts

#
eye 42

#
favorite 0

#
comment 0

We revise and extend the extreme value statistic, introduced in \cite{gup08}, to study directional dependence in the high redshift supernova data; arising either from departures from the cosmological principle or due to direction dependent statistical systematics in e data. We introduce a likelihood function that analytically marginalises over the Hubble constant, and use it to extend our previous statistic. We also introduce a new statistic that is sensitive to direction dependence arising...

Source: http://arxiv.org/abs/1005.2868v1

35
35

Sep 22, 2013
09/13

by
Varun Sahni; Tarun Deep Saini; A. A. Starobinsky; Ujjaini Alam

texts

#
eye 35

#
favorite 0

#
comment 0

We introduce a new cosmological diagnostic pair $\lbrace r,s\rbrace$ called Statefinder. The Statefinder is dimensionless and, like the Hubble and deceleration parameters $H(z)$ and $q(z)$, is constructed from the scale factor of the Universe and its derivatives only. The parameter $r(z)$ forms the next step in the hierarchy of geometrical cosmological parameters used to study the Universe after $H$ and $q$, while the parameter $s(z)$ is a linear combination of $q$ and $r$ chosen in such a way...

Source: http://arxiv.org/abs/astro-ph/0201498v2

49
49

Sep 19, 2013
09/13

by
Tarun Deep Saini

texts

#
eye 49

#
favorite 0

#
comment 0

We construct a simple, regularized estimator for the dark energy equation of state by using the recently introduced linear response approximation. We show that even a simple regularization substantially improves the performance of the free form fitting approach. The use of linear response approximation allows an analytic construction of maximum likelihood estimator, in a convenient and easy to use matrix form. We show that in principle, such regularized free form fitting can give us an unbiased...

Source: http://arxiv.org/abs/astro-ph/0302291v2

36
36

Sep 19, 2013
09/13

by
Ujjaini Alam; Varun Sahni; Tarun Deep Saini; A. A. Starobinsky

texts

#
eye 36

#
favorite 0

#
comment 0

The coming few years are likely to witness a dramatic increase in high quality Sn data as current surveys add more high redshift supernovae to their inventory and as newer and deeper supernova experiments become operational. Given the current variety in dark energy models and the expected improvement in observational data, an accurate and versatile diagnostic of dark energy is the need of the hour. This paper examines the Statefinder diagnostic in the light of the proposed SNAP satellite which...

Source: http://arxiv.org/abs/astro-ph/0303009v4

42
42

Sep 21, 2013
09/13

by
Tarun Deep Saini; Mamta Gulati; S. Sridhar

texts

#
eye 42

#
favorite 0

#
comment 0

Thin accretion discs around massive compact objects can support slow pressure modes of oscillations in the linear regime that have azimuthal wavenumber $m=1$. We consider finite, flat discs composed of barotropic fluid for various surface density profiles and demonstrate--through WKB analysis and numerical solution of the eigenvalue problem--that these modes are stable and have spatial scales comparable to the size of the disc. We show that the eigenvalue equation can be mapped to a...

Source: http://arxiv.org/abs/0901.4229v2

54
54

Sep 23, 2013
09/13

by
Shashikant Gupta; Tarun Deep Saini

texts

#
eye 54

#
favorite 0

#
comment 0

Two new statistics, namely $\Delta_\chi^2$ and $\Delta_\chi$, based on extreme value theory, were derived in \cite{gupta08,gupta10}. We use these statistics to study direction dependence in the HST key project data which provides the most precise measurement of the Hubble constant. We also study the non-Gaussianity in this data set using these statistics. Our results for $\Delta_\chi^2$ show that the significance of direction dependent systematics is restricted to well below one $\sigma$...

Source: http://arxiv.org/abs/1105.4949v1

49
49

Sep 20, 2013
09/13

by
Tarun Deep Saini; T. Padmanabhan; Sarah Bridle

texts

#
eye 49

#
favorite 0

#
comment 0

We show that the distance measures (such as the luminosity and angular diameter distances) are linear functionals of the equation-of-state function w(z) of the dark energy to a fair degree of accuracy in the regimes of interest. That is, the distance measures can be expressed as a sum of (i) a constant and (ii) an integral of a weighting function multiplied by the equation of state parameter w(z). The existence of such an accurate linear response approximation has several important...

Source: http://arxiv.org/abs/astro-ph/0301536v2

116
116

Jul 20, 2013
07/13

by
Ujjaini Alam; Varun Sahni; Tarun Deep Saini; A. A. Starobinsky

texts

#
eye 116

#
favorite 0

#
comment 0

In a recent paper (astro-ph/0311364) Alam et al argued that the SNe data of Tonry et al 2003 and Barris et al 2003 appear to favour DE which evolves in time, provided no other priors are invoked. (The effect of invoking priors such as the age of the Universe, the values of $H_0$ and $\Omega_m$ and CMB/LSS observations could modify this conclusion, as demonstrated in astro-ph/0403687 and other recent papers.) The approach adopted by Alam et al to reconstruct the properties of DE was severely...

Source: http://arxiv.org/abs/astro-ph/0406672v1

32
32

Sep 21, 2013
09/13

by
Ujjaini Alam; Varun Sahni; Tarun Deep Saini; A. A. Starobinsky

texts

#
eye 32

#
favorite 0

#
comment 0

We reconstruct the equation of state $w(z)$ of dark energy (DE) using a recently released data set containing 172 type Ia supernovae without assuming the prior $w(z) \geq -1$ (in contrast to previous studies). We find that dark energy evolves rapidly and metamorphoses from dust-like behaviour at high $z$ ($w \simeq 0$ at $z \sim 1$) to a strongly negative equation of state at present ($w \lleq -1$ at $z \simeq 0$). Dark energy metamorphosis appears to be a robust phenomenon which manifests for...

Source: http://arxiv.org/abs/astro-ph/0311364v3

46
46

Sep 22, 2013
09/13

by
Ritabrata Biswas; Subenoy Chakraborty; Tarun Deep Saini; Banibrata Mukhopadhyay

texts

#
eye 46

#
favorite 0

#
comment 0

We study the accretion of modified Chaplygin gas upon different types of black hole. Modified Chaplygin gas is one of the best candidates for a combined model of dark matter and dark energy. In addition, from a field theoretical point of view the modified Chaplygin gas model is equivalent to that of a scalar field having a self-interacting potential. We formulate the equations related to both spherical accretion and disc accretion, and respective winds. The corresponding numerical solutions of...

Source: http://arxiv.org/abs/1101.4602v1